It is the only interstate to traverse the whole north–south length of Washington, but the undivided highways U.S. Route 97 and U.S. Route 395 do also.[7][8] I-5 is also the only north–south primary interstate highway in Washington.[9][10] The road is the busiest in the state; an estimated 240,000 motorists use the road daily.[11] The second busiest is I-405 at 201,000.[12]

Interstate 5 has 7.14 miles (11.49 km)[3] of reversibleexpress lanes within Seattle. The express lanes begin after the Yesler Way bridge and tunnel under the northbound lanes of I-5 and intersect Cherry Street/Columbia Street. From the interchange, the express lanes intersect Mercer Street and then cross the Ship Canal Bridge. The express lanes are on the lower deck below the lanes of I-5. Near the end of the bridge is a reversible ramp that accesses Northeast 42nd Street. After the bridge ends, the express lanes continue northbound and intersect with Ravenna Boulevard, Lake City Way, and Northeast 103rd Street/1st Avenue Northeast. Finally, the lanes merge with I-5 in Northgate just north of the Northgate Mall.[5][30][32][34][35]

The express lanes are open to all traffic; however, some ramps are designated for HOV vehicles only to assist with traffic flow and mitigate constrained space. These ramps include Cherry Street/Columbia Street/5th Avenue, Ravenna Boulevard, and Pike Street/Downtown Seattle Transit Tunnel. The express lanes carry northbound traffic from 12pm to 11pm Monday through Friday and 2pm to 11pm Saturday and Sunday. They carry southbound traffic from 5 am to 11am Monday through Friday and from 7am to 1pm Saturday and Sunday.[36][37]

During the past two decades, parts of the interstate have been periodically inundated by floodwaters as a result of area storms. Floods in November 1990, February 1996, December 2007, and January 2009[61] each caused temporary closures of I-5 between the Kelso-Longview and Chehalis-Centralia areas. The causes of this problem, as noted by the United States Army Corps of Engineers, are attributable to "decades of clear-cut logging, modifications of waterways, and destruction of natural flood control features within the Chehalis River basin."[62]

The January 7, 2009 flood closed a 20-mile (32 km) section of the interstate for several days. The Washington State Department of Transportation was unable to offer a detour since all three main east/west passes were also closed due to severe mudslide and avalanche danger.

South of the Ship Canal Bridge Interstate 5 separates the Eastlake and Cascade neighborhoods from the Capitol Hill neighborhood, and separates Downtown Seattle from the Capitol Hill and First Hill neighborhoods. Its construction necessitated the demolition of significantly developed areas and cut off walking commutes to downtown for many First Hill residents, who "were by far the most vociferous critics of the proposed route,"[66] but far from the only ones. Architect Paul Thiry said in the early 1970s, "It was with the Freeway, cutting through the very heart of the city, that Seattle began taking one of its wrong turns and started to lose its identity as a city." He proposed a lid extending from Columbia Street north to Olive Way, roughly the entire length of downtown.[67]

Among the buildings torn down in the Downtown-First Hill area to build the freeway was the Hotel Kalmar at Sixth Avenue and James Street (built 1881 as the Western Hotel, demolished 1962), the last of Seattle's pioneer-era hotels, predating the Great Seattle Fire,[68][69] and Seattle's then-oldest public building, the Seventh Avenue Fire Hall (built 1890, demolished c. 1962).[66]

In the years since the freeway's construction, Seattle has made several efforts to stitch back together pedestrian routes disrupted by the freeway, achieving part of Thiry's proposed "lid". The most visible of these efforts are Freeway Park (opened 1976), built as a lid over the freeway and connecting Downtown to First Hill, and the Washington State Convention and Trade Center (built 1982-1988) adjacent to Freeway Park, also bridging the freeway.[66] The 7.5-acre (30,000 m2) I-5 Colonnademountain bike park (opened 2007) uses the freeway as a roof and reconnects Eastlake to Capitol Hill.[70]

On Interstate 5, the only planned improvement on the express lanes is a direct connection to the future SR 520 HOV lanes, to be constructed as part of the Evergreen Point Floating Bridge replacement project.

The portion of I-5 from US 2 / SR 529 Spur in Everett to SR 528 in Marysville opened at a ribbon-cutting ceremony on May 14, 1969 by Devvie Herivel, Miss Everett, and Katherine Smith, Miss Marysville. This was the last portion of Interstate 5 from California to Canada that was opened, free of traffic lights and draw bridges.[73] The stretch of I-5 from Marysville to Mount Vernon was already opened as a 4-lane divided highway without overpasses.[74]

^Washington State Legislature (1913). "An act relating to public highways, classifying the same and naming and fixing the routes of certain state roads.". Session Laws of the State of Washington. Olympia, WA: State of Washington. 1913 chapter 65, p. 221.: "A highway starting at the international boundary line at Blaine, Washington; thence southerly by the most feasible route through the cities of Bellingham, Mount Vernon, Everett, Seattle, Renton, along the easterly side of the White River Valley through Kent, Auburn, Tacoma, Olympia, Tenino, Centralia, Chehalis, to the southern boundary line at the city of Vancouver, Washington, to be known as The Pacific Highway."

^Washington State Legislature (1923). "An act relating to, classifying, naming and fixing the routes of certain state highways, amending Section 6796, and repealing Sections 6791, 6792, 6793, 6794, 6795, 6797, 6798, 6799, 6800, 6801, 6802, 6803, 6804, 6805, 6806, 6808, 6809, 6811, 6812, 6813 and 6816 of Remington's Compiled Statutes.". Session Laws of the State of Washington. Olympia, WA: State of Washington. 1923 chapter 185, p. 627-628.: "A primary state highway, to be known as State Road No. 1 or the Pacific Highway, is established as follows: Beginning at the international boundary line at Blaine in the County of Whatcom; thence by the most feasible route in a southerly direction through the cities of Bellingham, Mt. Vernon, Everett, Seattle, Tacoma, Olympia, Chehalis, Kelso and Vancouver to the interstate bridge over the Columbia River between Vancouver and Portland."

^Cameron 2004, pp. 331, 332, "On February 3, 1965, Rose Clare Menalo of Meadowdale High School, Miss Sno-King, cut the ribbon which opened the long awaited 19.7 miles of the Everett to Seattle section of the new I-5 freeway."

^Cameron 2004, p. 332, "It was still another year later when on May 14, 1969 Devvie Herivel, Miss Everett, and Katherine Smith, Miss Marysville, snipped the ribbon of the four-mile stretch across the Snohomish River flats to open a route free of traffic signals and draw bridges, finally linking Canada and California. Harold Walsh, state highways commissioner from Everett was master of ceremonies at the dedication taking place on the new Steamboat Slough bridge.

^Cameron 2004, p. 332, "North if Marysville, where there was already a divided highway, it was mostly the matter of building overpasses along Highway 99 at such places as Island Crossing, Lakewood, and Stimson Crossing."